1. Field of the Invention
The present invention relates to the field of permanent magnets. More specifically, the present invention relates to the field of a permanent magnet structure as may be embodied in a sputtering magnetron for generating an external magnetic field capable of saturating and penetrating a target material used in a sputtering process.
2. Description of the Related Art
Introduction
Sputtering is a process in which atoms of a target material are deposited on a substrate. In general, the sputtering process involves loading clean substrates into a vacuum chamber, evacuating the chamber to a low pressure to remove contaminants, filling the chamber with an inert gas, (such as argon), to a particular operating pressure, initiating and sustaining a glow discharge, or plasma, by application of a high negative electrostatic voltage to a cathode/target assembly, and sputter depositing of the target material onto the substrates.
When a sufficiently large DC voltage is applied to the sputtering system, electrons are pulled from the target material towards the substrates. As the target electrons move from the target material toward the substrates, many such electrons strike argon gas atoms. This strips electrons from the argon atoms, leaving positively charged argon ions and liberating additional electrons. The positively charged argon ions accelerate toward the surface of the negative target and gain kinetic energy. Energy is transferred to the target surface upon impact and neutral atoms of the target material are ejected as a result. Target atoms expend energy as they travel from the target surface, so the substrates are generally located nearby, where bombardment energy causes the target atoms to deposit on, and diffuse into, the substrate surface. Electrons are also freed from the target surface and accelerate into the plasma where they strike argon gas atoms to create more ions and free electrons to sustain the process.
In an improvement to the sputtering system, the external magnetic field of a magnet assembly, called a magnetron, is superimposed upon the electric field to speed up the sputtering process. The external magnetic field traps electrons liberated from the target by applying a magnetomotive force to direct the electrons back to the surface of the negative target where they are repelled. The resulting path of the electrons becomes convoluted, i.e., a spiral path through the argon gas. This increases the frequency of strikes with argon atoms which creates more argon ions to strike the target surface and transfer target material from the target to the substrates at a faster rate. The arrangement of magnetic and electric fields in the sputtering system is known to those of ordinary skill in the art as a cross field device.
It should be noted that ion strikes on the target are distributed around the center line of the charged particle path. This causes a characteristic gaussian erosion pattern normal to the path of the charged particle. The center of the erosion pattern, i.e., the greatest erosion depth, occurs where the magnetic neutral zone is found. It is at this point that the magnetic field vector component parallel to the target surface is at a maximum.
If the strength of the magnetic field vector component parallel to the target surface is increased, the sputter rate increases up to an asymptotic level. Thus, there is a limit to the useful intensity of the applied magnetic field, depending on particle velocity due to electrostatic field strength, sputter system pressure, and other well known physical parameters of the sputtering system.
If the external magnetic field generated by the sputtering magnetron can be spread so the magnetic field vector component parallel to the target surface (normal to the electric field) covers a larger width, or area, then the erosion pattern on the target surface will spread providing more of the available target material for use in the sputtering system. This is relevant to the economy of the sputtering system operation. Such a result can be accomplished by spreading the effective magnetic poles and/or superimposing the magnetic fields of several dipole magnets so their external magnetic field components add, or buck, and shape the external magnetic field. However, the magnetic field vector component parallel to the target surface will generally be greatest close at the midpoint between magnetic poles, but to a lesser degree.
Where the target material in a sputtering system is magnetically permeable, magnetron influenced sputtering occurs if, and when an external magnetic field saturates and passes through the target material. The external magnetic field will pass through the target material if the target material offers a low reluctance path, and in a manner inversely proportional to the reluctance of available magnetic flux paths, or if the magnetron supplies sufficient magnetic flux to saturate the target. Generally, permanent magnets with high residual induction are utilized to provide sufficient magnetic flux.
Permeable magnetic materials have a characteristic "S" shaped permeability versus magnetization curve. Since reluctance is proportional to permeability, the lowest reluctance will occur when the magnetic material is pushed into saturation. However, there may be favorable low reluctance flux paths in that portion of the ferrous target material operating in its initial permeability range. Permeable materials also produce induction, i.e., a magnetic field of their own, when their magnetic domains are aligned by an external magnetic field, e.g., the dipole field of a magnetron.
Thus, the magnetic field of the magnetron is attracted to, shunted and shaped by the location, geometry, and magnetic properties of ferrous target materials. This causes problems when attempting to shape the vector components of the magnetic field contributing to the aforementioned cross field device.